A direct current (DC), voltage regulator regulates the supply voltage to a preferred, accurate, and stable amplitude while supplying a large current to drive an external circuit load. The regulated voltage should be highly stable and accurate even when the supply voltage drifts and the circuit load change drastically.
Voltage regulation is essential in many applications. For example, a wireless, radio frequency (RF) telephone is typically operated with a battery capable of generating a raw voltage between 2.7 to 5.5 volts, depending upon its state of discharge. This battery supplies power to both the antenna load, when transmitting, and to circuits such as a voltage controlled oscillator (VCO). Because the battery voltage changes as the battery discharges and the transmission load can vary dynamically, the current draw on the battery may vary widely while the telephone is being used. Current draw within the range of 1 mA to 100 mA is common.
A VCO generates a frequency in response to an applied voltage signal. Since each frequency, within the range of frequencies, that a VCO may generate is linearly proportional to an applied voltage, the VCO is very sensitive to fluctuations of the voltage supply. A highly stable reference voltage is needed to prevent the VCO frequency from varying in response to fluctuations of the battery voltage.
A bandgap reference is useful in many applications because it provides a substantially invariant voltage when subjected to variations of temperature and power supply voltage. Voltage regulation is typically achieved by generating a bandgap voltage and applying this voltage to a resistive chain. At an electrical tap point between the resistive elements of the chain, the preferred amplitude of the voltage is obtained and this serves as the reference supply. Resistors of the resistive chain are selectively chosen to generate the desired voltage amplitude at the tap point.
FIG. 1 illustrates a block diagram representation of a prior art design for a voltage regulator. This voltage regulator is comprised of a bandgap reference circuit 11, a voltage divider, and a feedback amplifier. The bandgap reference voltage is applied to one input of a differential amplifier 14 and a fractional portion of the regulated voltage is applied to the other input, through a MOSFET 15 and resistor 12. The regulated voltage provided by this design is given by the equation: V.sub.R =(V.sub.BG *(R1 +R2))/R1, where V.sub.BG is the bandgap voltage, R1 is the value of the resistance element 12, and R2 is the value of resistance element 13.
FIG. 2 illustrates a prior art circuit configuration for implementing the voltage regulator represented in FIG. 1. Here, the bandgap reference voltage is generated at the collector of transistor 21 and is equal to the combined voltage drop across resistor 31 and the base-emitter voltage, V.sub.be, of transistor 21. The regulated voltage is generated by the resistive chain of resistors 23 and 24 in conjunction with P-MOS transistor 26 and is used as a power source for the bandgap reference circuitry, as well as an external load. An emitter-coupled pair of transistors, 27 and 28, form a differential feedback amplifier used to modulate the current conducted by the drain-source junction of transistor 26. By modulating the drain-source current of transistor 26 in response to the amplitude difference between the bandgap reference voltage and the portion of the output reference voltage dropped across resistor 24, it is possible to maintain a constant DC voltage potential at the regulated voltage terminal, V.sub.reg. A constant, regulated voltage potential may be sustained even if the supply voltage drifts or the current changes in response to load variations.
To achieve a highly accurate voltage potential across resistor 24 and at the output of the regulator (i.e., good power supply rejection), the feedback amplifier must have a large gain. With prior art designs, it is difficult to obtain both a large gain and a high degree of stability for the feedback amplifier. Increases in gain are realized through modifications that cause concomitant decreases in stability, and vice versa. The gain of the differential amplifier may be increased by increasing the value of resistor 30. However, the increased magnitude of resistor 30 causes a phase-gain pole, at the gate of transistor 26, to move to a lower frequency. By moving the phase-gain pole to a lower frequency, the voltage regulator's stability is degraded drastically. Use of a current mirror, from the gain stage to the output transistor 26, will not overcome the problem when the voltage regulator is used to provide power to an external device having large load variations.